In general, eye care professionals (ECPs) prescribe lenses, fit lenses, and dispense prescriptions for eye correction to improve vision and to diagnose and treat various eye maladies. Lens prescriptions are typically determined using a refractor-head or test lenses that are positioned perpendicular to a patient's lines of sight. However, when the patient chooses a frame and the frame is fitted to the patient, the lenses are usually tilted based on the position of the frame. This tilt of the lenses can result in the prescription power being altered.
With the introduction of free form surfacing, also known as direct or digital surfacing, prescription lenses can be customized to a particular patient. Free form surfacing allows for complex surface shapes to be produced using computer numerically controlled (CNC) cutting tools and polishing machines.
Through the use of free form surfacing combined with accurate position of wear measurements, lenses can be customized to the patient's prescription, fitting geometry, and frame information. The position of wear is the position of the lens relative to the physical features of the actual patient. The position of wear is commonly measured in terms of vertex distance, pupillary distance, frame wrap, and pantoscopic tilt.
These measurements are generally taken using manual devices such as rulers, protractor type devices, distometers, and other manual devices or digital measurement devices. Current digital measurement devices are generally large stand alone devices that require the use of a separate reference sensor. In general, the current digital measurement devices require the patient's frames to be fit to the patient and the reference sensor must be positioned correctly on the fitted frames. If the reference sensor is not correctly positioned on the fitted frames, the measurement accuracy can be compromised.